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This option is the Linux counterpart to the
CYGNUM_REDBOOT_FIS_DIRECTORY_BLOCK RedBoot compile time
option.

The option specifies which Flash sectors holds the RedBoot
partition table. A zero or positive value gives an absolute
erase block number. A negative value specifies a number of
sectors before the end of the device.

For example "2" means block number 2, "-1" means the last
block and "-2" means the penultimate block.